1. Field of the Invention
The present invention relates to a laser beam scanner for use in a code reader or the like adapted to optically read a code such as a bar code by scanning the same with a laser beam.
2. Discussion of the Prior Art
Conventionally, code readers have been used which are adapted to optically read a code by scanning with a laser beam a code surface on which a code such as a bar code or a character is formed, and by receiving the light reflected from the code surface by a light-receiving element. An output of the light-receiving element becomes a signal corresponding to the relative strength of the reflected light. In a bar code composed of black bars and white spaces, for instance, a small signal is obtained with respect to a bar, while a large signal is obtained with respect to a space. Accordingly, it is possible to obtain a binary signal corresponding to a bar code if the output of the light-receiving element is, for instance, amplified and is then binarized by being discriminated on the basis of an appropriate slice level.
Although He-Ne lasers were used in the past as light sources for generating laser beams, semiconductor lasers have come to be used in recent years in an attempt to make the overall apparatus compact and lightweight. Since the laser light emitted from a semiconductor laser diffuses remarkably, the output light is normally narrowed down by means of a lens or the like, thereby obtaining a substantially parallel laser beam. For instance, in a case where the code reader is used for reading bar codes, if the bar code has a bar width of 0.2 mm or less, in order to enable the resolution of such a fine-width bar code, it is necessary to narrow the laser light in such a manner that the beam diameter becomes 0.2 mm or less. For this reason, the laser beam used for a bar code reader or the like is not completely parallel light but convergent light having a focal point.
If such a laser beam is used, reading with high resolution is possible in the vicinity of a focal point where the beam diameter is small. In a case where the reading of a code is effected at a position distant from the focal point, the resolution declines. Accordingly, there unavoidably arises the problem that high resolution cannot be ensured over a wide range of reading distance.
A first prior art which is directed to this problem is disclosed in, for instance, Japanese Patent Application Unexamined Publication No. Hei 2-7182, and its basic arrangement is shown in FIG. 6 hereof. Light from a semiconductor laser light source 1 is condensed by a condenser lens 2, thereby forming a laser beam 3. The laser beam 3 has a beam waist BW at a focal position at a distance FL determined by the semiconductor laser light source 1 and the condenser lens 2. If the reading of, for instance, a bar code is effected at the position of this beam waist, reading with a maximum resolution is possible. In this prior art, the arrangement is provided such that the condenser lens 2 is displaced in a direction 4 along the optical axis thereof, whereby the distance FL from the semiconductor laser light source 1 to the focal position is shortened, as shown in FIG. 6(a), or lengthened, as shown in FIG. 6(b).
According to such an arrangement, if the beam waist BW is formed at a position in the vicinity of the code surface in correspondence with a distance from an apparatus body to the code surface where the bar code is formed, the reading of the bar code with high resolution is realized irrespective of the reading distance. As a result, satisfactory reading of a bar code becomes possible in a wide reading range.
A second prior art which is directed to the above-described problem is disclosed in Japanese Patent Application Unexamined Publication No. Hei 2-133891. In this prior art, there are a plurality of laser beam emitting means each constituted by a semiconductor laser light source and a lens, the positions of focal points of the laser beam emitting means are set at mutually different reading distances. This plurality of beam emitting means are used by selecting a beam corresponding to a particular reading distance In such an arrangement as well, it is possible to change the position of the beam waist of the laser beam in correspondence with the reading distance, so that the reading of a code with high resolution can be attained over a wide range of reading.
A third prior art directed to the above-described problem is shown in FIG. 7. A laser beam 12 from a beam emitting means 11 is reflected by a mirror 13A or 13B in its optical path, to a polygon mirror 15 rotatively driven at a constant speed in the direction of arrow 14, and is introduced to a code surface 16. Since the advancing direction of the laser 12 after its reflection changes due to teh rotation of the polygon mirror, the code surface 16 is automatically scanned.
The reflecting mirror 13B which is closer to the beam emitting means 11 is moved into or out of the optical path of the laser beam 12 by an unillustrated driving mechanism, whereby a laser beam 12 can be reflected by the reflecting mirror 13A or by reflecting mirror 13B. As a result, the optical path of the laser beam 12 has two lengths, and thus there are two distances from the apparatus body to a focal position where the beam waist is formed. It goes without saying that if two or more reflecting mirrors are arranged to be insertable between the beam emitting means and the reflecting mirror 13B, more changes in the optical path length become possible. With such an arrangement as well, the reading of codes with high resolution becomes possible in correspondence with various reading distances.
In addition, an arrangement may be alternatively provided such that by modifying the arrangement shown in FIG. 7, one reflecting mirror is used, and this mirror is displaceable from the position of the reflecting mirror 13A to the position of the reflecting mirror 13B. With such an arrangement as well, the position where the beam waist is formed is variable, so that it is possible to operate with various reading distances.
In accordance with the above-described first prior art, the distance FL to the focal position is changed by changing the positional relationship between the semiconductor laser light source 1 and the condenser lens 2. For this reason, in a case where the condenser lens 2 is brought into proximity to the semiconductor laser light source 1 to lengthen the distance FL as shown in FIG. 6(b), the angle .theta. at which the laser beam is emitted from the semiconductor laser light source 1 to enter the condenser lens 2 becomes large, so that the beam diameter at the beam waist BW becomes large. In other words, if the distance FL is increased, as shown in FIG. 8, the beam diameter at the beam waist BW increases substantially in proportion to the same. As a result, it becomes impossible to read at a large distance with high resolution.
In addition, in accordance with the above-described second prior art, if an attempt is made to change the focal position in a multiplicity of stages, a multiplicity of beam emitting means including semiconductor laser light sources and lenses are required, resulting in a higher cost. In addition, there is another problem in that a complicated optical arrangement is necessary to ensure that the optical paths of the laser beams from the plurality of beam emitting means coincide with each other.
Furthermore, in accordance with the above-described third prior art, if an attempt is made to change the focal position in, for instance, three stages, a mechanical arrangement for driving two reflecting mirrors is necessary, and a driving mechanism is generally required for individually driving reflecting mirrors and for allowing these reflecting mirrors to be selectively moved into or out of the optical path of the laser beam 12. For this reason, the number of mechanical arrangements increases, so that this prior art is not suitable for adjusting the optical path length in a multiplicity of stages.
In addition, in the arrangement in which one reflecting mirror is continuously placed in the optical path of the laser beam 12, since the operating distance of the reflecting mirror is long, its response characteristic is poor, so that this arrangement is not suitable for practical use.